While recent automobiles have been made higher in power, the automobiles have been required to exhibit silence. Moreover, the use environment of automobiles is also various, and the automobiles may be used in districts high in ambient temperature, such as Southeast Asia and Middle East countries, and may be used in cold districts. In general, as for automobiles having a vibration damping rubber which contains natural rubber as a main component and which is improved in heat resistance, problems are not especially caused when the automobiles are used in districts high in ambient temperature. However, in the case of allowing the automobiles to stand still in cold districts for a long term, abnormal vibrations may be generated when their engine is re-started. A reason therefor would be as follows: when their vibration damping rubber is exposed to a very low temperature (for example, about −30° C.), the dynamic spring constant of the rubber increases so that the function of the rubber as a vibration damping rubber is remarkably deteriorated. Accordingly, in consideration of the silence of automobiles, it is desired in the actual circumstances to develop a rubber composition for vibration damping rubbers which is a rubber showing a small variation in dynamic spring constant particularly at low temperatures.
As described above, in general, as a rubber component for a rubber composition for vibration damping rubbers, a rubber composition containing natural rubber as a main component has been hitherto used. As a technique of decreasing a variation in the dynamic spring constant of a vulcanized rubber of this rubber component-containing rubber composition, known is a technique of increasing the amount of a sulfur-based vulcanizing agent in the rubber composition. However, this technique tends to deteriorate the heat resistance of the vibration damping rubbers. Thus, this technique makes it difficult to make the heat resistance compatible with the restraint of a change, with time, in the dynamic spring constant in the range of low temperatures.
Patent Document 1 listed below describes a technique of blending, in a rubber composition, a rubber component, an amine-ketone-based antiaging agent, a mixed resin, stearic acid and zinc oxide in order that the resultant composition can be used for tire treads and be improved in gripping performance, abrasion resistance and external appearance properties.